Recently, a demand has been rapidly increased for a clean liquid fuel with a low sulfur content. In response to this demand, various methods for producing clean fuels have already been studied in the fuel oil industry. Due to a regulation requiring diesel fuel in particular to contain 10 ppm by mass or less of sulfur, petroleum companies have, therefore, created systems to produce a clean fuel with improved catalysts or installation of more facilities.
The main base oil of diesel fuel is generally a gas oil fraction distilling from an atmospheric distillation tower or a cracker. Production of a low sulfur content clean diesel fuel thus requires the sulfur to be removed with a hydrorefining unit.
Typically, gas oil is hydrorefined in a hydrogen flow in a fixed-bed reactor filled with a desulfurization catalyst under high temperature and pressure conditions.
Conventionally, a catalyst comprising an active metal component selected from Groups VIA and VIII in the periodic table supported on a support of a porous inorganic oxide such as alumina, alumina-silica, titania, or alumina-titania has been widely used as a catalyst used for the purpose of hydrotreating a hydrocarbon oil.
The titania support is known to exhibit a higher desulfurization performance than the alumina support but has a problem that it has a small specific surface area and low thermal stability at elevated temperatures. A titania-containing catalyst is known to be produced by a process wherein a titania support is prepared using titania gel (see Patent Literature 1 below) or wherein an alumina-titania support is prepared by loading a water-soluble titania compound on an alumina support (see Patent Literature 2 below). The catalyst described in Patent Literature 1 contains a large amount of titania, which is expensive and increase the production cost and bulk density compared with a catalyst comprising a conventional alumina support. Since the catalyst described in Patent Literature 2 can support titania only in an amount corresponding the water absorption rate of titania, it is expensive to be produced industrially because loading step is necessarily repeated so as to load titania in a large amount on a support. A method has been proposed wherein titania is mixed and highly dispersed in alumina upon preparation of thereof (see Patent Literature 3). This method enables titania to be highly dispersed in alumina but has disadvantages that as the content of titania increases, crystallization thereof is likely to accelerate, resulting in a catalyst with a decreased specific surface area and a deteriorated sharpness of the pore distribution. Furthermore, the catalyst is not a catalyst having sufficient properties that can cope with a regulation of 10 ppm sulfur content.
As described above, an improvement in a desulfurization catalyst has been vigorously carried out to produce a clean fuel. Despite of research and development for a long period of time, a catalyst technology satisfying a higher desulfurization activity has not been achieved yet. It may be because a plurality of desulfurization passages and active site structure effective therefor are not clear and variation in support composition affects differently desulfurization activity.